In the past, various attempts have gone into improving a tread design for a tread portion to prevent abnormal wears, such as uneven or sloped wear or shoulder wear, in the tread portion of a pneumatic radial tire for heavy load used in trucks, buses, light trucks, etc.
It is, however, the present state of the art that any of the conventional attempts falls under the category of such a tread design which affords the tread portion in a bisymmetrical form with the tread equator plane as the center, as shown in FIG. 1.
On the other hand, when a tire is used while being actually put on a vehicle, it shows different wear rates on the right and left side with the tread equator plane as the center, namely, on the inner and outer side of the vehicle. Particularly, a tire mounted on the front wheel side (i.e., on the steering wheel side) gives a faster wear rate of the tread portion on the outer side of the vehicle than that on the inner side thereof due to influences caused by turning at a corner or the like.
Therefore, the conventional tire having a bisymmetrical tread design with the tread equator plane as the center results in a larger wear rate on the outer side of the tread portion than that on the inner side thereof, particularly when mounted on the steering wheel side, due to influences caused by turning at a corner or the like. FIG. 2 illustrates the abnormal wear thus resulted, and this is one of primary factors of less wear resistance of tires. For example, the service life of such a tire will expire at the time when the grooves on the outer side are completely gone because of the abnormal wear.